Generally, the knives used in this type of application are mounted on a shaft and spaced according to the desired cutting width. These knives are known generally as "upper-knives". This knife-shaft is mounted opposite to a second shaft parallel to the first one, and on which are mounted lower knives against which the upper knives are urged into contact.
It is a common practice to arrange the knives in such a manner, in order to manufacture photographic films or soft metal webs and particularly to cut webs adapted to record magnetic sounds. During use, such webs are guided on both sides, along their edges, when passing on the recorder heads, and in this case, parallelism defects should not exceed some hundredths of a millimeter.
When cutting the materials into webs, the knives often wear away very quickly and, when the wear becomes too significant, the knives cut by compression, thus giving webs with very uneven or curved edges and with a significant dusting level. The knives must then be replaced and reground.
It is obvious that the frequent removal and reinsertion of the knives highly decrease the productivity of the cutting devices using such knives.
Up to now, various cutting devices have been used. Thus, the french patent 2 439 736 discloses a cutting device including a lower knife-shaft and an upper knife-shaft, which are parallel between them, and both driven and inserted into stationary bearings.
The lower knives comprise plane front faces rigidly flanged axially on the lower knife-shaft. The corresponding upper knives, mounted on the upper knife-shaft, come into engagement between these front faces. The upper knives are each formed by assembling a hub on which are mounted a spacer ring centered in the length direction with, on both sides, a resilient plane annular knife pressed on an inner collar, these elements being pressed against one another by means of spring discs. According to this patent, the spacer height corresponds approximately to the resilient knife height. However, when such knives are ground, which seems difficult to perform without removing them from the shaft, the knife height is necessarily reduced, thus implying also that the spacer height is reduced. Necessarily, this consumes time and this is not a very economical solution, a simpler but still less economical solution consisting in changing the knives each time some degree of wear is reached.
The U.S. Pat. No. 3,286,574 discloses a cutting device in which upper and lower knives are flexible discs mounted on two parallel shafts, the upper knives being urged into contact, by the edge of one of their faces, with the lower knives. Such a system exhibits drawbacks as, during disc grinding, the height of these discs is necessarily reduced, and their flexibility is altered, and then it is necessary to modify the respective positioning of the shafts, thus leading to a more complicated device. In addition, there are matching problems between the upper and the lower knives, particularly when the knife-shafts contain a lot of discs located side by side according to a pitch which must be as regular as possible, in order that all the upper knives will be simultaneously urged into contact with the lower knives. Thus, positioning errors of the order of a few microns on each knife will involve, at the end of the shaft, an error of a few tenths of a millimeter. And as the grinding of such discs is performed on the disc edge, it is not possible to correct these errors in a simple way. Moreover, such a device is not adapted for significant web widths, such as those used for photographic materials.
There are also other types of knives, generally used for this kind of application. Among them, are the so called "shoulder knives". These knives, which will be described with more detail hereinafter, are formed of discs having a central hole, in order to be mounted on a cutting shaft while having a single freedom degree, the translation, and comprising at the periphery of one of their faces, a shoulder adapted to be urged into contact with a lower knife, also mounted on a shaft parallel to the first one. The pressure of the upper knives on the lower knives is applied by means of spring devices or other mechanical devices arranged at the back of each knife. Such devices, by reason of mechanical tolerances, create problems concerning the control of the pressure exerted by the upper knife on the lower knife, thus involving an uneven wear of the shoulder at the periphery of the upper knife. Necessarily, it results in variations in the cut web's width. The resulting dusting level on the cutting edges is significant, thus unfavorably affecting the quality of the resulting webs. Moreover, such knives are difficult to grind directly on the shaft, and, by reason of the complex mounting mechanism of these knives, the grinding operation consumes much time. Finally, these knives are-heavy, which is a significant drawback for a cutting shaft on which up to 100 knives and more, can be mounted.